Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On 2019/3/15 上午3:33, Andrea Arcangeli wrote:
Hello Jason,
On Thu, Mar 14, 2019 at 09:49:03PM +0800, Jason Wang wrote:
Yes since we don't want to slow down 32bit.
If you've a lot of ram there's no justification to stick to a 32bit
kernel, so I don't think there's need to maintain a separate model
just for 32bit. I really wouldn't care about the performance of 32bit
with >700MB of RAM if that would cause any maintenance burden. Let's
focus on the best 64bit implementation that will work equally
optimally on 32bit with <= 700M of RAM.
Yes, but probably there are still some reasons of keeping copy_user()
friends as a fallback. When we have a large virtqueue, the ring may
occupy more than one page. This means the VA might not be contiguous
when using kmap(). Instead of doing tricks in the accessories, maybe
it's or simpler better just fall back to copy_user() in this case. And
we meet the similar issue when software device IOTLB is used for vhost.
And in the following example for gup, we can simply do a fallback when
we race with the invalidation.
Michael also tends to keep the copy_user(), he suggested to use
copy_user() for VIVT archs then there's no need for a explicit
flush_dcache_page(). And he also want a module parameter for falling
back to copy_user() for e.g debugging purpose.
Talking to Jerome about the set_page_dirty issue, he raised the point
of what happens if two thread calls a mmu notifier invalidate
simultaneously. The first mmu notifier could call set_page_dirty and
then proceed in try_to_free_buffers or page_mkclean and then the
concurrent mmu notifier that arrives second, then must not call
set_page_dirty a second time.
With KVM sptes mappings and vhost mappings you would call
set_page_dirty (if you invoked gup with FOLL_WRITE) only when
effectively tearing down any secondary mapping (you've got pointers in
both cases for the mapping). So there's no way to risk a double
set_page_dirty from concurrent mmu notifier invalidate because the
invalidate takes a lock when it has to teardown the mapping and so
set_page_dirty is only run in the first invalidate method and not in
the second. In the spte case even better, as you wouldn't need to call
it even at teardown time unless the spte is dirty (if shadow mask
allows dirty sptes with EPT2 or NPT or shadow MMU).
I see, the sounds indeed better.
If you instead had to invalidate a secondary MMU mapping that isn't
tracked by the driver (again: not vhost nor KVM case), you could have
used the dirty bit of the kernel pagetable to call set_page_dirty and
disambiguate but that's really messy, and it would prevent the use of
gigapages in the direct mapping too and it'd require vmap for 4k
tracking.
To make sure set_page_dirty is run a single time no matter if the
invalidate known when a mapping is tear down, I suggested the below
model:
access = FOLL_WRITE
repeat:
page = gup_fast(access)
put_page(page) /* need a way to drop FOLL_GET from gup_fast instead! */
spin_lock(mmu_notifier_lock);
if (race with invalidate) {
spin_unlock..
goto repeat;
}
if (access == FOLL_WRITE)
set_page_dirty(page)
establish writable mapping in secondary MMU on page
spin_unlock
(replace spin_lock with mutex_lock for vhost of course if you stick to
a mutex and _start/_end instead of non-sleepable ->invalidate_range)
Yes, I probably stick to the vq mutex since the invalidation needs to be
synchronized with the whole packet processing routine.
"race with invalidate" is the usual "mmu_notifier_retry" in kvm_host.h
to be implemented for vhost.
We could add a FOLL_DIRTY flag to add to FOLL_TOUCH to move the
set_page_dirty inside GUP forced (currently it's not forced if the
linux pte is already dirty). And we could remove FOLL_GET.
Then if you have the ability to disambiguate which is the first
invalidate that tears down a mapping to any given page (vhost can do
that trivially, it's just a pointer to a page struct to kmap), in the
mmu notifier invalidate just before dropping the spinlock you would
do this check:
def vhost_invalidate_range_start():
[..]
spin_lock(mmu_notifier_lock);
[..]
if (vhost->page_pointer) {
if (access == FOLL_WRITE)
VM_WARN_ON(!PageDirty(vhost->page_pointer));
vhost->page_pointer = NULL;
/* no put_page, already done at gup time */
}
spin_unlock(..
Generally speaking set_page_dirty is safer done after the last
modification to the data of the page. However the way stable page
works, as long as the mmu notifier invalidate didn't run, the PG_dirty
cannot go away.
Ok.
So this model solves the issue with guaranteeing a single
set_page_dirty is run before page_mkclean or try_to_free_buffers can
run, even for drivers that implement the invalidate as a generic "TLB
flush" across the whole secondary MMU and that cannot disambiguate the
first invalidate from a second invalidate if they're issued
concurrently on the same address
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
Hello Jason,
On Thu, Mar 14, 2019 at 09:49:03PM +0800, Jason Wang wrote:
> Yes since we don't want to slow down 32bit.
If you've a lot of ram there's no justification to stick to a 32bit
kernel, so I don't think there's need to maintain a separate model
just for 32bit. I really wouldn't care about the performance of 32bit
with >700MB of RAM if that would cause any maintenance burden. Let's
focus on the best 64bit implementation that will work equally
optimally on 32bit with <= 700M of RAM.
Talking to Jerome about the set_page_dirty issue, he raised the point
of what happens if two thread calls a mmu notifier invalidate
simultaneously. The first mmu notifier could call set_page_dirty and
then proceed in try_to_free_buffers or page_mkclean and then the
concurrent mmu notifier that arrives second, then must not call
set_page_dirty a second time.
With KVM sptes mappings and vhost mappings you would call
set_page_dirty (if you invoked gup with FOLL_WRITE) only when
effectively tearing down any secondary mapping (you've got pointers in
both cases for the mapping). So there's no way to risk a double
set_page_dirty from concurrent mmu notifier invalidate because the
invalidate takes a lock when it has to teardown the mapping and so
set_page_dirty is only run in the first invalidate method and not in
the second. In the spte case even better, as you wouldn't need to call
it even at teardown time unless the spte is dirty (if shadow mask
allows dirty sptes with EPT2 or NPT or shadow MMU).
If you instead had to invalidate a secondary MMU mapping that isn't
tracked by the driver (again: not vhost nor KVM case), you could have
used the dirty bit of the kernel pagetable to call set_page_dirty and
disambiguate but that's really messy, and it would prevent the use of
gigapages in the direct mapping too and it'd require vmap for 4k
tracking.
To make sure set_page_dirty is run a single time no matter if the
invalidate known when a mapping is tear down, I suggested the below
model:
access = FOLL_WRITE
repeat:
page = gup_fast(access)
put_page(page) /* need a way to drop FOLL_GET from gup_fast instead! */
spin_lock(mmu_notifier_lock);
if (race with invalidate) {
spin_unlock..
goto repeat;
}
if (access == FOLL_WRITE)
set_page_dirty(page)
establish writable mapping in secondary MMU on page
spin_unlock
(replace spin_lock with mutex_lock for vhost of course if you stick to
a mutex and _start/_end instead of non-sleepable ->invalidate_range)
"race with invalidate" is the usual "mmu_notifier_retry" in kvm_host.h
to be implemented for vhost.
We could add a FOLL_DIRTY flag to add to FOLL_TOUCH to move the
set_page_dirty inside GUP forced (currently it's not forced if the
linux pte is already dirty). And we could remove FOLL_GET.
Then if you have the ability to disambiguate which is the first
invalidate that tears down a mapping to any given page (vhost can do
that trivially, it's just a pointer to a page struct to kmap), in the
mmu notifier invalidate just before dropping the spinlock you would
do this check:
def vhost_invalidate_range_start():
[..]
spin_lock(mmu_notifier_lock);
[..]
if (vhost->page_pointer) {
if (access == FOLL_WRITE)
VM_WARN_ON(!PageDirty(vhost->page_pointer));
vhost->page_pointer = NULL;
/* no put_page, already done at gup time */
}
spin_unlock(..
Generally speaking set_page_dirty is safer done after the last
modification to the data of the page. However the way stable page
works, as long as the mmu notifier invalidate didn't run, the PG_dirty
cannot go away.
So this model solves the issue with guaranteeing a single
set_page_dirty is run before page_mkclean or try_to_free_buffers can
run, even for drivers that implement the invalidate as a generic "TLB
flush" across the whole secondary MMU and that cannot disambiguate the
first invalidate from a second invalidate if they're issued
concurrently on the same address by two different CPUs.
So for those drivers that can disambiguate trivially (like KVM and
vhost) we'll just add a debug check in the invalidate to validate the
common code for all mmu notifier users.
This is the solution for RDMA hardware and everything that can support
mmu notifiers too and they can take infinitely long secondary MMU
mappins without interfering with stable pages at all (i.e. long term
pins but without pinning) perfectly safe and transparent to the whole
stable page code.
I think O_DIRECT for stable pages shall be solved taking the page lock
or writeback lock or a new rwsem in the inode that is taken for
writing by page_mkclean and try_to_free_buffers and for reading by
outstanding O_DIRECT in flight I/O, like I suggested probably ages ago
but then we only made GUP take the page pin, which is fine for mmu
notifier actually (except those didn't exist back then). To solve
O_DIRECT we can leverage the 100% guarantee that the pin will be
dropped ASAP and stop page_mkclean and stop or trylock in
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On 2019/3/14 下午6:42, Michael S. Tsirkin wrote: Which means after we fix vhost to add the flush_dcache_page after kunmap, Parisc will get a double hit (but it also means Parisc was the only one of those archs needed explicit cache flushes, where vhost worked correctly so far.. so it kinds of proofs your point of giving up being the safe choice). What double hit? If there's no cache to flush then cache flush is a no-op. It's also a highly piplineable no-op because the CPU has the L1 cache within easy reach. The only event when flush takes a large amount time is if we actually have dirty data to write back to main memory. I've heard people complaining that on some microarchitectures even no-op cache flushes are relatively expensive. Don't ask me why, but if we can easily avoid double flushes we should do that. It's still not entirely free for us. Our internal cache line is around 32 bytes (some have 16 and some have 64) but that means we need 128 flushes for a page ... we definitely can't pipeline them all. So I agree duplicate flush elimination would be a small improvement. James I suspect we'll keep the copyXuser path around for 32 bit anyway - right Jason? Yes since we don't want to slow down 32bit. Thanks So we can also keep using that on parisc... -- ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On Wed, Mar 13, 2019 at 09:37:08AM -0700, James Bottomley wrote: > On Wed, 2019-03-13 at 09:05 -0700, Christoph Hellwig wrote: > > On Tue, Mar 12, 2019 at 01:53:37PM -0700, James Bottomley wrote: > > > I've got to say: optimize what? What code do we ever have in the > > > kernel that kmap's a page and then doesn't do anything with it? You > > > can > > > guarantee that on kunmap the page is either referenced (needs > > > invalidating) or updated (needs flushing). The in-kernel use of > > > kmap is > > > always > > > > > > kmap > > > do something with the mapped page > > > kunmap > > > > > > In a very short interval. It seems just a simplification to make > > > kunmap do the flush if needed rather than try to have the users > > > remember. The thing which makes this really simple is that on most > > > architectures flush and invalidate is the same operation. If you > > > really want to optimize you can use the referenced and dirty bits > > > on the kmapped pte to tell you what operation to do, but if your > > > flush is your invalidate, you simply assume the data needs flushing > > > on kunmap without checking anything. > > > > I agree that this would be a good way to simplify the API. Now > > we'd just need volunteers to implement this for all architectures > > that need cache flushing and then remove the explicit flushing in > > the callers.. > > Well, it's already done on parisc ... I can help with this if we agree > it's the best way forward. It's really only architectures that > implement flush_dcache_page that would need modifying. > > It may also improve performance because some kmap/use/flush/kunmap > sequences have flush_dcache_page() instead of > flush_kernel_dcache_page() and the former is hugely expensive and > usually unnecessary because GUP already flushed all the user aliases. > > In the interests of full disclosure the reason we do it for parisc is > because our later machines have problems even with clean aliases. So > on most VIPT systems, doing kmap/read/kunmap creates a fairly harmless > clean alias. Technically it should be invalidated, because if you > remap the same page to the same colour you get cached stale data, but > in practice the data is expired from the cache long before that > happens, so the problem is almost never seen if the flush is forgotten. > Our problem is on the P9xxx processor: they have a L1/L2 VIPT L3 PIPT > cache. As the L1/L2 caches expire clean data, they place the expiring > contents into L3, but because L3 is PIPT, the stale alias suddenly > becomes the default for any read of they physical page because any > update which dirtied the cache line often gets written to main memory > and placed into the L3 as clean *before* the clean alias in L1/L2 gets > expired, so the older clean alias replaces it. > > Our only recourse is to kill all aliases with prejudice before the > kernel loses ownership. > > > > > Which means after we fix vhost to add the flush_dcache_page after > > > > kunmap, Parisc will get a double hit (but it also means Parisc > > > > was the only one of those archs needed explicit cache flushes, > > > > where vhost worked correctly so far.. so it kinds of proofs your > > > > point of giving up being the safe choice). > > > > > > What double hit? If there's no cache to flush then cache flush is > > > a no-op. It's also a highly piplineable no-op because the CPU has > > > the L1 cache within easy reach. The only event when flush takes a > > > large amount time is if we actually have dirty data to write back > > > to main memory. > > > > I've heard people complaining that on some microarchitectures even > > no-op cache flushes are relatively expensive. Don't ask me why, > > but if we can easily avoid double flushes we should do that. > > It's still not entirely free for us. Our internal cache line is around > 32 bytes (some have 16 and some have 64) but that means we need 128 > flushes for a page ... we definitely can't pipeline them all. So I > agree duplicate flush elimination would be a small improvement. > > James I suspect we'll keep the copyXuser path around for 32 bit anyway - right Jason? So we can also keep using that on parisc... -- MST ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On Tue, Mar 12, 2019 at 01:53:37PM -0700, James Bottomley wrote: > I've got to say: optimize what? What code do we ever have in the > kernel that kmap's a page and then doesn't do anything with it? You can > guarantee that on kunmap the page is either referenced (needs > invalidating) or updated (needs flushing). The in-kernel use of kmap is > always > > kmap > do something with the mapped page > kunmap > > In a very short interval. It seems just a simplification to make > kunmap do the flush if needed rather than try to have the users > remember. The thing which makes this really simple is that on most > architectures flush and invalidate is the same operation. If you > really want to optimize you can use the referenced and dirty bits on > the kmapped pte to tell you what operation to do, but if your flush is > your invalidate, you simply assume the data needs flushing on kunmap > without checking anything. I agree that this would be a good way to simplify the API. Now we'd just need volunteers to implement this for all architectures that need cache flushing and then remove the explicit flushing in the callers.. > > Which means after we fix vhost to add the flush_dcache_page after > > kunmap, Parisc will get a double hit (but it also means Parisc was > > the only one of those archs needed explicit cache flushes, where > > vhost worked correctly so far.. so it kinds of proofs your point of > > giving up being the safe choice). > > What double hit? If there's no cache to flush then cache flush is a > no-op. It's also a highly piplineable no-op because the CPU has the L1 > cache within easy reach. The only event when flush takes a large > amount time is if we actually have dirty data to write back to main > memory. I've heard people complaining that on some microarchitectures even no-op cache flushes are relatively expensive. Don't ask me why, but if we can easily avoid double flushes we should do that. ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On Tue, Mar 12, 2019 at 03:02:54PM -0700, James Bottomley wrote:
> I'm sure there must be workarounds elsewhere in the other arch code
> otherwise things like this, which appear all over drivers/, wouldn't
> work:
>
> drivers/scsi/isci/request.c:1430
>
> kaddr = kmap_atomic(page);
> memcpy(kaddr + sg->offset, src_addr, copy_len);
> kunmap_atomic(kaddr);
>
Are you sure "page" is an userland page with an alias address?
sg->page_link = (unsigned long)virt_to_page(addr);
page_link seems to point to kernel memory.
I found an apparent solution like parisc on arm 32bit:
void __kunmap_atomic(void *kvaddr)
{
unsigned long vaddr = (unsigned long) kvaddr & PAGE_MASK;
int idx, type;
if (kvaddr >= (void *)FIXADDR_START) {
type = kmap_atomic_idx();
idx = FIX_KMAP_BEGIN + type + KM_TYPE_NR * smp_processor_id();
if (cache_is_vivt())
__cpuc_flush_dcache_area((void *)vaddr, PAGE_SIZE);
However on arm 64bit kunmap_atomic is not implemented at all and other
32bit implementations don't do it, for example sparc seems to do the
cache flush too if the kernel is built with CONFIG_DEBUG_HIGHMEM
(which makes the flushing conditional to the debug option).
The kunmap_atomic where fixmap is used, is flushing the tlb lazily so
even on 32bit you can't even be sure if there was a tlb flush for each
single page you unmapped, so it's hard to see how the above can work
safe, is "page" would have been an userland page mapped with aliased
CPU cache.
> the sequence dirties the kernel virtual address but doesn't flush
> before doing kunmap. There are hundreds of other examples which is why
> I think adding flush_kernel_dcache_page() is an already lost cause.
In lots of cases kmap is needed to just modify kernel memory not to
modify userland memory (where get/put_user is more commonly used
instead..), there's no cache aliasing in such case.
> Actually copy_user_page() is unused in the main kernel. The big
> problem is copy_user_highpage() but that's mostly highly optimised by
> the VIPT architectures (in other words you can fiddle with kmap without
> impacting it).
copy_user_page is not unused, it's called precisely by
copy_user_highpage, which is why the cache flushes are done inside
copy_user_page.
static inline void copy_user_highpage(struct page *to, struct page *from,
unsigned long vaddr, struct vm_area_struct *vma)
{
char *vfrom, *vto;
vfrom = kmap_atomic(from);
vto = kmap_atomic(to);
copy_user_page(vto, vfrom, vaddr, to);
kunmap_atomic(vto);
kunmap_atomic(vfrom);
}
___
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On Tue, Mar 12, 2019 at 02:19:15PM -0700, James Bottomley wrote:
> I mean in the sequence
>
> flush_dcache_page(page);
> flush_dcache_page(page);
>
> The first flush_dcache_page did all the work and the second it a
> tightly pipelined no-op. That's what I mean by there not really being
> a double hit.
Ok I wasn't sure it was clear there was a double (profiling) hit on
that function.
void flush_kernel_dcache_page_addr(void *addr)
{
unsigned long flags;
flush_kernel_dcache_page_asm(addr);
purge_tlb_start(flags);
pdtlb_kernel(addr);
purge_tlb_end(flags);
}
#define purge_tlb_start(flags) spin_lock_irqsave(_tlb_lock, flags)
#define purge_tlb_end(flags)spin_unlock_irqrestore(_tlb_lock, flags)
You got a system-wide spinlock in there that won't just go away the
second time. So it's a bit more than a tightly pipelined "noop".
Your logic of adding the flush on kunmap makes sense, all I'm saying
is that it's sacrificing some performance for safety. You asked
"optimized what", I meant to optimize away all the above quoted code
that will end running twice for each vhost set_bit when it should run
just once like in other archs. And it clearly paid off until now
(until now it run just once and it was the only safe one).
Before we can leverage your idea to flush the dcache on kunmap in
common code without having to sacrifice performance in arch code, we'd
need to change all other archs to add the cache flushes on kunmap too,
and then remove the cache flushes from the other places like copy_page
or we'd waste CPU. Then you'd have the best of both words, no double
flush and kunmap would be enough.
Thanks,
Andrea
___
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On Tue, Mar 12, 2019 at 01:53:37PM -0700, James Bottomley wrote:
> I've got to say: optimize what? What code do we ever have in the
> kernel that kmap's a page and then doesn't do anything with it? You can
> guarantee that on kunmap the page is either referenced (needs
> invalidating) or updated (needs flushing). The in-kernel use of kmap is
> always
>
> kmap
> do something with the mapped page
> kunmap
>
> In a very short interval. It seems just a simplification to make
> kunmap do the flush if needed rather than try to have the users
> remember. The thing which makes this really simple is that on most
> architectures flush and invalidate is the same operation. If you
> really want to optimize you can use the referenced and dirty bits on
> the kmapped pte to tell you what operation to do, but if your flush is
> your invalidate, you simply assume the data needs flushing on kunmap
> without checking anything.
Except other archs like arm64 and sparc do the cache flushing on
copy_to_user_page and copy_user_page, not on kunmap.
#define copy_user_page(to,from,vaddr,pg) __cpu_copy_user_page(to, from, vaddr)
void __cpu_copy_user_page(void *kto, const void *kfrom, unsigned long vaddr)
{
struct page *page = virt_to_page(kto);
copy_page(kto, kfrom);
flush_dcache_page(page);
}
#define copy_user_page(to, from, vaddr, page) \
do {copy_page(to, from);\
sparc_flush_page_to_ram(page); \
} while (0)
And they do nothing on kunmap:
static inline void kunmap(struct page *page)
{
BUG_ON(in_interrupt());
if (!PageHighMem(page))
return;
kunmap_high(page);
}
void kunmap_high(struct page *page)
{
unsigned long vaddr;
unsigned long nr;
unsigned long flags;
int need_wakeup;
unsigned int color = get_pkmap_color(page);
wait_queue_head_t *pkmap_map_wait;
lock_kmap_any(flags);
vaddr = (unsigned long)page_address(page);
BUG_ON(!vaddr);
nr = PKMAP_NR(vaddr);
/*
* A count must never go down to zero
* without a TLB flush!
*/
need_wakeup = 0;
switch (--pkmap_count[nr]) {
case 0:
BUG();
case 1:
/*
* Avoid an unnecessary wake_up() function call.
* The common case is pkmap_count[] == 1, but
* no waiters.
* The tasks queued in the wait-queue are guarded
* by both the lock in the wait-queue-head and by
* the kmap_lock. As the kmap_lock is held here,
* no need for the wait-queue-head's lock. Simply
* test if the queue is empty.
*/
pkmap_map_wait = get_pkmap_wait_queue_head(color);
need_wakeup = waitqueue_active(pkmap_map_wait);
}
unlock_kmap_any(flags);
/* do wake-up, if needed, race-free outside of the spin lock */
if (need_wakeup)
wake_up(pkmap_map_wait);
}
static inline void kunmap(struct page *page)
{
}
because they already did it just above.
> > Which means after we fix vhost to add the flush_dcache_page after
> > kunmap, Parisc will get a double hit (but it also means Parisc was
> > the only one of those archs needed explicit cache flushes, where
> > vhost worked correctly so far.. so it kinds of proofs your point of
> > giving up being the safe choice).
>
> What double hit? If there's no cache to flush then cache flush is a
> no-op. It's also a highly piplineable no-op because the CPU has the L1
> cache within easy reach. The only event when flush takes a large
> amount time is if we actually have dirty data to write back to main
> memory.
The double hit is in parisc copy_to_user_page:
#define copy_to_user_page(vma, page, vaddr, dst, src, len) \
do { \
flush_cache_page(vma, vaddr, page_to_pfn(page)); \
memcpy(dst, src, len); \
flush_kernel_dcache_range_asm((unsigned long)dst, (unsigned long)dst +
len); \
} while (0)
That is executed just before kunmap:
static inline void kunmap(struct page *page)
{
flush_kernel_dcache_page_addr(page_address(page));
}
Can't argue about the fact your "safer" kunmap is safer, but we cannot
rely on common code unless we remove some optimization from the common
code abstractions and we make all archs do kunmap like parisc.
Thanks,
Andrea
___
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On Tue, Mar 12, 2019 at 08:46:50AM -0700, James Bottomley wrote: > On Tue, 2019-03-12 at 07:54 -0400, Michael S. Tsirkin wrote: > > On Tue, Mar 12, 2019 at 03:17:00PM +0800, Jason Wang wrote: > > > > > > On 2019/3/12 ä¸å11:52, Michael S. Tsirkin wrote: > > > > On Tue, Mar 12, 2019 at 10:59:09AM +0800, Jason Wang wrote: > [...] > > > At least for -stable, we need the flush? > > > > > > > > > > Three atomic ops per bit is way to expensive. > > > > > > > > > Yes. > > > > > > Thanks > > > > See James's reply - I stand corrected we do kunmap so no need to > > flush. > > Well, I said that's what we do on Parisc. The cachetlb document > definitely says if you alter the data between kmap and kunmap you are > responsible for the flush. It's just that flush_dcache_page() is a no- > op on x86 so they never remember to add it and since it will crash > parisc if you get it wrong we finally gave up trying to make them. > > But that's the point: it is a no-op on your favourite architecture so > it costs you nothing to add it. Yes, the fact Parisc gave up and is doing it on kunmap is reasonable approach for Parisc, but it doesn't move the needle as far as vhost common code is concerned, because other archs don't flush any cache on kunmap. So either all other archs give up trying to optimize, or vhost still has to call flush_dcache_page() after kunmap. Which means after we fix vhost to add the flush_dcache_page after kunmap, Parisc will get a double hit (but it also means Parisc was the only one of those archs needed explicit cache flushes, where vhost worked correctly so far.. so it kinds of proofs your point of giving up being the safe choice). Thanks, Andrea ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On Tue, Mar 12, 2019 at 03:17:00PM +0800, Jason Wang wrote:
>
> On 2019/3/12 上午11:52, Michael S. Tsirkin wrote:
> > On Tue, Mar 12, 2019 at 10:59:09AM +0800, Jason Wang wrote:
> > > On 2019/3/12 上午2:14, David Miller wrote:
> > > > From: "Michael S. Tsirkin"
> > > > Date: Mon, 11 Mar 2019 09:59:28 -0400
> > > >
> > > > > On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote:
> > > > > > On 2019/3/8 下午10:12, Christoph Hellwig wrote:
> > > > > > > On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote:
> > > > > > > > This series tries to access virtqueue metadata through kernel
> > > > > > > > virtual
> > > > > > > > address instead of copy_user() friends since they had too much
> > > > > > > > overheads like checks, spec barriers or even hardware feature
> > > > > > > > toggling. This is done through setup kernel address through
> > > > > > > > vmap() and
> > > > > > > > resigter MMU notifier for invalidation.
> > > > > > > >
> > > > > > > > Test shows about 24% improvement on TX PPS. TCP_STREAM doesn't
> > > > > > > > see
> > > > > > > > obvious improvement.
> > > > > > > How is this going to work for CPUs with virtually tagged caches?
> > > > > > Anything different that you worry?
> > > > > If caches have virtual tags then kernel and userspace view of memory
> > > > > might not be automatically in sync if they access memory
> > > > > through different virtual addresses. You need to do things like
> > > > > flush_cache_page, probably multiple times.
> > > > "flush_dcache_page()"
> > >
> > > I get this. Then I think the current set_bit_to_user() is suspicious, we
> > > probably miss a flush_dcache_page() there:
> > >
> > >
> > > static int set_bit_to_user(int nr, void __user *addr)
> > > {
> > > unsigned long log = (unsigned long)addr;
> > > struct page *page;
> > > void *base;
> > > int bit = nr + (log % PAGE_SIZE) * 8;
> > > int r;
> > >
> > > r = get_user_pages_fast(log, 1, 1, );
> > > if (r < 0)
> > > return r;
> > > BUG_ON(r != 1);
> > > base = kmap_atomic(page);
> > > set_bit(bit, base);
> > > kunmap_atomic(base);
> > > set_page_dirty_lock(page);
> > > put_page(page);
> > > return 0;
> > > }
> > >
> > > Thanks
> > I think you are right. The correct fix though is to re-implement
> > it using asm and handling pagefault, not gup.
>
>
> I agree but it needs to introduce new helpers in asm for all archs which is
> not trivial.
We can have a generic implementation using kmap.
> At least for -stable, we need the flush?
>
>
> > Three atomic ops per bit is way to expensive.
>
>
> Yes.
>
> Thanks
See James's reply - I stand corrected we do kunmap so no need to flush.
--
MST
___
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On 2019/3/12 下午3:51, Jason Wang wrote:
On 2019/3/12 下午1:14, James Bottomley wrote:
On Tue, 2019-03-12 at 10:59 +0800, Jason Wang wrote:
On 2019/3/12 上午2:14, David Miller wrote:
From: "Michael S. Tsirkin"
Date: Mon, 11 Mar 2019 09:59:28 -0400
On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote:
On 2019/3/8 下午10:12, Christoph Hellwig wrote:
On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote:
This series tries to access virtqueue metadata through
kernel virtual
address instead of copy_user() friends since they had too
much
overheads like checks, spec barriers or even hardware
feature
toggling. This is done through setup kernel address through
vmap() and
resigter MMU notifier for invalidation.
Test shows about 24% improvement on TX PPS. TCP_STREAM
doesn't see
obvious improvement.
How is this going to work for CPUs with virtually tagged
caches?
Anything different that you worry?
If caches have virtual tags then kernel and userspace view of
memory
might not be automatically in sync if they access memory
through different virtual addresses. You need to do things like
flush_cache_page, probably multiple times.
"flush_dcache_page()"
I get this. Then I think the current set_bit_to_user() is suspicious,
we
probably miss a flush_dcache_page() there:
static int set_bit_to_user(int nr, void __user *addr)
{
unsigned long log = (unsigned long)addr;
struct page *page;
void *base;
int bit = nr + (log % PAGE_SIZE) * 8;
int r;
r = get_user_pages_fast(log, 1, 1, );
if (r < 0)
return r;
BUG_ON(r != 1);
base = kmap_atomic(page);
set_bit(bit, base);
kunmap_atomic(base);
This sequence should be OK. get_user_pages() contains a flush which
clears the cache above the user virtual address, so on kmap, the page
is coherent at the new alias. On parisc at least, kunmap embodies a
flush_dcache_page() which pushes any changes in the cache above the
kernel virtual address back to main memory and makes it coherent again
for the user alias to pick it up.
It would be good if kmap()/kunmap() can do this but looks like we can
not assume this? For example, sparc's flush_dcache_page()
Sorry, I meant kunmap_atomic().
Thanks
doesn't do flush_dcache_page(). And bio_copy_data_iter() do
flush_dcache_page() after kunmap_atomic().
Thanks
James
___
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On 2019/3/12 下午1:14, James Bottomley wrote:
On Tue, 2019-03-12 at 10:59 +0800, Jason Wang wrote:
On 2019/3/12 上午2:14, David Miller wrote:
From: "Michael S. Tsirkin"
Date: Mon, 11 Mar 2019 09:59:28 -0400
On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote:
On 2019/3/8 下午10:12, Christoph Hellwig wrote:
On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote:
This series tries to access virtqueue metadata through
kernel virtual
address instead of copy_user() friends since they had too
much
overheads like checks, spec barriers or even hardware
feature
toggling. This is done through setup kernel address through
vmap() and
resigter MMU notifier for invalidation.
Test shows about 24% improvement on TX PPS. TCP_STREAM
doesn't see
obvious improvement.
How is this going to work for CPUs with virtually tagged
caches?
Anything different that you worry?
If caches have virtual tags then kernel and userspace view of
memory
might not be automatically in sync if they access memory
through different virtual addresses. You need to do things like
flush_cache_page, probably multiple times.
"flush_dcache_page()"
I get this. Then I think the current set_bit_to_user() is suspicious,
we
probably miss a flush_dcache_page() there:
static int set_bit_to_user(int nr, void __user *addr)
{
unsigned long log = (unsigned long)addr;
struct page *page;
void *base;
int bit = nr + (log % PAGE_SIZE) * 8;
int r;
r = get_user_pages_fast(log, 1, 1, );
if (r < 0)
return r;
BUG_ON(r != 1);
base = kmap_atomic(page);
set_bit(bit, base);
kunmap_atomic(base);
This sequence should be OK. get_user_pages() contains a flush which
clears the cache above the user virtual address, so on kmap, the page
is coherent at the new alias. On parisc at least, kunmap embodies a
flush_dcache_page() which pushes any changes in the cache above the
kernel virtual address back to main memory and makes it coherent again
for the user alias to pick it up.
It would be good if kmap()/kunmap() can do this but looks like we can
not assume this? For example, sparc's flush_dcache_page() doesn't do
flush_dcache_page(). And bio_copy_data_iter() do flush_dcache_page()
after kunmap_atomic().
Thanks
James
___
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On 2019/3/12 上午11:52, Michael S. Tsirkin wrote:
On Tue, Mar 12, 2019 at 10:59:09AM +0800, Jason Wang wrote:
On 2019/3/12 上午2:14, David Miller wrote:
From: "Michael S. Tsirkin"
Date: Mon, 11 Mar 2019 09:59:28 -0400
On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote:
On 2019/3/8 下午10:12, Christoph Hellwig wrote:
On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote:
This series tries to access virtqueue metadata through kernel virtual
address instead of copy_user() friends since they had too much
overheads like checks, spec barriers or even hardware feature
toggling. This is done through setup kernel address through vmap() and
resigter MMU notifier for invalidation.
Test shows about 24% improvement on TX PPS. TCP_STREAM doesn't see
obvious improvement.
How is this going to work for CPUs with virtually tagged caches?
Anything different that you worry?
If caches have virtual tags then kernel and userspace view of memory
might not be automatically in sync if they access memory
through different virtual addresses. You need to do things like
flush_cache_page, probably multiple times.
"flush_dcache_page()"
I get this. Then I think the current set_bit_to_user() is suspicious, we
probably miss a flush_dcache_page() there:
static int set_bit_to_user(int nr, void __user *addr)
{
unsigned long log = (unsigned long)addr;
struct page *page;
void *base;
int bit = nr + (log % PAGE_SIZE) * 8;
int r;
r = get_user_pages_fast(log, 1, 1, );
if (r < 0)
return r;
BUG_ON(r != 1);
base = kmap_atomic(page);
set_bit(bit, base);
kunmap_atomic(base);
set_page_dirty_lock(page);
put_page(page);
return 0;
}
Thanks
I think you are right. The correct fix though is to re-implement
it using asm and handling pagefault, not gup.
I agree but it needs to introduce new helpers in asm for all archs
which is not trivial. At least for -stable, we need the flush?
Three atomic ops per bit is way to expensive.
Yes.
Thanks
___
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On Tue, Mar 12, 2019 at 10:59:09AM +0800, Jason Wang wrote:
>
> On 2019/3/12 上午2:14, David Miller wrote:
> > From: "Michael S. Tsirkin"
> > Date: Mon, 11 Mar 2019 09:59:28 -0400
> >
> > > On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote:
> > > > On 2019/3/8 下午10:12, Christoph Hellwig wrote:
> > > > > On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote:
> > > > > > This series tries to access virtqueue metadata through kernel
> > > > > > virtual
> > > > > > address instead of copy_user() friends since they had too much
> > > > > > overheads like checks, spec barriers or even hardware feature
> > > > > > toggling. This is done through setup kernel address through vmap()
> > > > > > and
> > > > > > resigter MMU notifier for invalidation.
> > > > > >
> > > > > > Test shows about 24% improvement on TX PPS. TCP_STREAM doesn't see
> > > > > > obvious improvement.
> > > > > How is this going to work for CPUs with virtually tagged caches?
> > > >
> > > > Anything different that you worry?
> > > If caches have virtual tags then kernel and userspace view of memory
> > > might not be automatically in sync if they access memory
> > > through different virtual addresses. You need to do things like
> > > flush_cache_page, probably multiple times.
> > "flush_dcache_page()"
>
>
> I get this. Then I think the current set_bit_to_user() is suspicious, we
> probably miss a flush_dcache_page() there:
>
>
> static int set_bit_to_user(int nr, void __user *addr)
> {
> unsigned long log = (unsigned long)addr;
> struct page *page;
> void *base;
> int bit = nr + (log % PAGE_SIZE) * 8;
> int r;
>
> r = get_user_pages_fast(log, 1, 1, );
> if (r < 0)
> return r;
> BUG_ON(r != 1);
> base = kmap_atomic(page);
> set_bit(bit, base);
> kunmap_atomic(base);
> set_page_dirty_lock(page);
> put_page(page);
> return 0;
> }
>
> Thanks
I think you are right. The correct fix though is to re-implement
it using asm and handling pagefault, not gup.
Three atomic ops per bit is way to expensive.
--
MST
___
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On 2019/3/12 上午2:14, David Miller wrote:
From: "Michael S. Tsirkin"
Date: Mon, 11 Mar 2019 09:59:28 -0400
On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote:
On 2019/3/8 下午10:12, Christoph Hellwig wrote:
On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote:
This series tries to access virtqueue metadata through kernel virtual
address instead of copy_user() friends since they had too much
overheads like checks, spec barriers or even hardware feature
toggling. This is done through setup kernel address through vmap() and
resigter MMU notifier for invalidation.
Test shows about 24% improvement on TX PPS. TCP_STREAM doesn't see
obvious improvement.
How is this going to work for CPUs with virtually tagged caches?
Anything different that you worry?
If caches have virtual tags then kernel and userspace view of memory
might not be automatically in sync if they access memory
through different virtual addresses. You need to do things like
flush_cache_page, probably multiple times.
"flush_dcache_page()"
I get this. Then I think the current set_bit_to_user() is suspicious, we
probably miss a flush_dcache_page() there:
static int set_bit_to_user(int nr, void __user *addr)
{
unsigned long log = (unsigned long)addr;
struct page *page;
void *base;
int bit = nr + (log % PAGE_SIZE) * 8;
int r;
r = get_user_pages_fast(log, 1, 1, );
if (r < 0)
return r;
BUG_ON(r != 1);
base = kmap_atomic(page);
set_bit(bit, base);
kunmap_atomic(base);
set_page_dirty_lock(page);
put_page(page);
return 0;
}
Thanks
___
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
From: "Michael S. Tsirkin" Date: Mon, 11 Mar 2019 09:59:28 -0400 > On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote: >> >> On 2019/3/8 下午10:12, Christoph Hellwig wrote: >> > On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote: >> > > This series tries to access virtqueue metadata through kernel virtual >> > > address instead of copy_user() friends since they had too much >> > > overheads like checks, spec barriers or even hardware feature >> > > toggling. This is done through setup kernel address through vmap() and >> > > resigter MMU notifier for invalidation. >> > > >> > > Test shows about 24% improvement on TX PPS. TCP_STREAM doesn't see >> > > obvious improvement. >> > How is this going to work for CPUs with virtually tagged caches? >> >> >> Anything different that you worry? > > If caches have virtual tags then kernel and userspace view of memory > might not be automatically in sync if they access memory > through different virtual addresses. You need to do things like > flush_cache_page, probably multiple times. "flush_dcache_page()" ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote: > > On 2019/3/8 下午10:12, Christoph Hellwig wrote: > > On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote: > > > This series tries to access virtqueue metadata through kernel virtual > > > address instead of copy_user() friends since they had too much > > > overheads like checks, spec barriers or even hardware feature > > > toggling. This is done through setup kernel address through vmap() and > > > resigter MMU notifier for invalidation. > > > > > > Test shows about 24% improvement on TX PPS. TCP_STREAM doesn't see > > > obvious improvement. > > How is this going to work for CPUs with virtually tagged caches? > > > Anything different that you worry? If caches have virtual tags then kernel and userspace view of memory might not be automatically in sync if they access memory through different virtual addresses. You need to do things like flush_cache_page, probably multiple times. > I can have a test but do you know any > archs that use virtual tag cache? sparc I believe. > Thanks ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On 2019/3/8 下午10:12, Christoph Hellwig wrote: On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote: This series tries to access virtqueue metadata through kernel virtual address instead of copy_user() friends since they had too much overheads like checks, spec barriers or even hardware feature toggling. This is done through setup kernel address through vmap() and resigter MMU notifier for invalidation. Test shows about 24% improvement on TX PPS. TCP_STREAM doesn't see obvious improvement. How is this going to work for CPUs with virtually tagged caches? Anything different that you worry? I can have a test but do you know any archs that use virtual tag cache? Thanks ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote: > This series tries to access virtqueue metadata through kernel virtual > address instead of copy_user() friends since they had too much > overheads like checks, spec barriers or even hardware feature > toggling. This is done through setup kernel address through vmap() and > resigter MMU notifier for invalidation. > > Test shows about 24% improvement on TX PPS. TCP_STREAM doesn't see > obvious improvement. How is this going to work for CPUs with virtually tagged caches? ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
